1. Field of the Invention
This invention relates to a method of strengthening the alkali resistance of a porous glass and more particularly, it is concerned with a method of strengthening the alkali resistance of a porous glass of SiO.sub.2 by applying a ZrO.sub.2 coating.
2. Description of the Prior Art
A porous glass is prepared by utilizing the phase separation phenomenon of sodium borosilicate glass, i.e. by subjecting sodium borosilicate glass to a phase separation heat treatment to split into an SiO.sub.2 rich phase and B.sub.2 O.sub.3 --Na.sub.2 O rich phase, treating with an acid to remove the acid-soluble B.sub.2 O.sub.3 --Na.sub.2 O phase and to retain the acid-insoluble SiO.sub.2 phase as skeleton and thus forming a three-dimensionally connected network of SiO.sub.2. Processes for the production of such a porous glass are disclosed in "Makushi" 4 (4) 221-227, "Gel Permeation Chromatography of Macroparticles" 3-18 (1980), U.S. Pat. Nos. 3,549,524 (1970) and 3,758,284 (1973).
On the other hand, as another process for producing a porous glass, there has been proposed a technique of forming an oxide polymer based on the hydrolysis reaction of metal alkoxides. For example, the use of titanium tetralkoxide Ti(OR).sub.4 is described in "Bosei Kanri" 7, 23-25 (1979), which hydrolysis steps are represented below: EQU Ti(OR).sub.4 +H.sub.2 O.fwdarw.Ti(OR).sub.3 OH+ROH EQU Ti(OR).sub.3 OH+Ti(OR).sub.4 .fwdarw.(RO).sub.3 TiOTi(OR).sub.3 +ROH
This dimer is further hydrolysed and the hydrolysis proceeds substantially up to formation of titanium oxide, as follows: ##STR1##
In addition, "Yogyo Kyokaishi" 85 (9), 448-454 (1977) describes a method of forming an amorphous film consisting of a composite oxide polymer of TiO.sub.2 --ZrO.sub.2 --SiO.sub.2 based on the hydrolysis reaction of metal alcoholates.
However, the porous glass prepared as described above has the disadvantage that it tends to be chemically etched with an alkaline solution according to the following mechanism:
A porous glass consists of about 90% by weight or more of SiO.sub.2 which intrinsically tends to be chemically etched with an alkaline solution. This SiO.sub.2 forms a skeleton, i.e. matrix phase in such a small thickness, e.g. about several hundred .ANG. that it is readily subject to etching to be water-soluble sodium silicate as follows: EQU SiO.sub.2 +2NaOH.fwdarw.Na.sub.2 SiO.sub.3.nH.sub.2 O+(1-n)H.sub.2 O
Furthermore, a porous glass has a very large specific surface area as can readily be assumed from the formation scheme of phase separation. For example, the specific surface area amounts to about 200 m.sup.2 /g when the average pore size is 100 .ANG. in diameter. Such a physical shape causes increasing of etching with an alkaline solution.
For porous glasses, it is required to handle or treat alkaline solutions and in this case, the following drawbacks are brought. When a granular porous glass is used as a column packing material for gel permeation, ion exchange or adsorption chromatography in the separation and purification of various proteins, for example, impure proteins adsorb and remain often on the packing material after the separation and purification of the object material. If the packing material is treated with a strongly alkaline solution for the purpose of desorption thereof, the packing material itself is etched and size-reduced to decrease the effective volume of the packing phase. When a granular porous glass is used as an immobilized enzyme support desorportion of the deactivated enzyme is sometimes carried out by treating with an alkaline solution after the reaction of the enzyme and substrate and in this case, the immobilized phase is also etched to give a similar phenomenon to described above.